1. Field of the Invention
The present invention relates to an image forming apparatus, an image stabilization control method and an image stabilization control program product, and more particularly to an image forming apparatus, an image stabilization control method and an image stabilization control program product correcting tone to obtain desired image density.
2. Description of the Related Art
In an image forming apparatus adapted to electrophotography, as image forming processing is affected by change over time of a developer or a photoconductor, environmental change such as temperature and humidity variation, and the like, it is difficult to maintain image density constant. Accordingly, in the image forming apparatus, in order to obtain desired image density, namely, in order to stabilize the image to be formed, regardless of the state of the developer, the photoconductor and peripherals, image forming conditions such as a development bias potential, a laser exposure amount and the like have been controlled. In addition, in the image forming apparatus, control for correcting the tone is carried out such that input tone matches with output image density. Such control is referred to as image stabilization control.
In controlling tone correction in the image forming apparatus, a detector detects a toner pattern representing a tone correction pattern created on an image carrier, and the density of the toner pattern is determined based on a voltage value representing a result of detection. As a method used to control tone correction, a method of selecting an optimal tone correction table from a plurality of tables used for tone correction (hereinafter, referred to as the tone correction table) stored in the image forming apparatus based on the determined density of the toner pattern has been known. In addition, a method of fitting a selected tone correction table based on the density of the toner pattern has also been known. The tone correction table will be described later.
In addition, a method of calculating a tone correction table directly from determined density of the toner pattern has also been known. As such a method, Japanese Laid-Open Patent Publication No. 2005-091736 (hereinafter, Patent Document 1) discloses a method of calculating a tone correction table by combining linear approximation and curve approximation. In addition, Japanese Laid-Open Patent Publication No. 2004-206137 (hereinafter, Patent Document 2) discloses a method of calculating a tone correction table by performing linear approximation of a detection value greater than a reference value.
The image forming apparatus employing a belt as the image carrier, for example, of a type adapted to a tandem process having an intermediate transfer belt, includes an intermediate transfer belt for primary transfer of a toner image as will be described later. The intermediate transfer belt is an endless belt suspended by a roller at least two portions without loosening, and the belt rotates at a prescribed speed as a result of rotation of the roller.
The portion of the intermediate transfer belt suspended in the longitudinal direction by the roller is bent at an acute angle. Therefore, for example, if the belt is left for a long time without rotating or the like, deformation due to suspension by the roller (hereinafter, such deformation is referred to as creep) is caused at that portion.
The toner pattern transferred to the intermediate transfer belt is detected by a detector that receives a reflected component (reflected light) from a surface of the belt. Therefore, at the creep portion, since a distance between the surface of the belt and the detector, an angle between the surface of the belt and the detector, and the like are varied as a result of deformation of the belt shape, the toner pattern is not appropriately detected.
Such creep is eliminated after the image forming processing is performed and a driven state lasts for some time. Normally, however, image stabilization control is carried out, for example, when the image forming apparatus is turned on after it is left for a long time in a non-driven state or when printing or the like is instructed. Namely, image stabilization control is carried out when it is likely that creep has been caused in the intermediate transfer belt. Accordingly, if the method as disclosed in Patent Documents 1 and 2 is used in image stabilization control to detect the toner pattern transferred onto the intermediate transfer belt in image stabilization control, the result of detection tends to contain noise due to creep. The noise due to creep is more likely when density of the toner pattern is higher or the reflected component from the surface of the belt is less, than when density of the toner pattern is lower or the reflected component is greater. In correcting the tone in image stabilization control, as the toner pattern of high density to the toner pattern of low density are detected, image stabilization control is susceptible to creep. If image stabilization control is carried out using the detection results containing noise, the image forming apparatus cannot form an image of desired image density and a quality of the formed image becomes lower.
In addition to creep, abnormality of the shape on the intermediate transfer belt, for example, breakage such as flaw on the intermediate transfer belt or deformation due to deterioration or the like, may become factors of noise, as in the case of creep. Therefore, the toner pattern is not appropriately detected either in such a case, which leads to lower quality of the formed image.